Structural Biochemistry/The Endosymbiotic Theory

The endosymbiotic theory deals with the origins of mitochondria and chloroplasts, two eukaryotic organelles that have bacteria characteristics. Mitochondria and chloroplasts are believed to have developed from symbiotic bacteria, specifically alpha-proteobacteria and cyanobacteria, respectively. The theory states that a prokaryotic cell was consumed or engulfed by a larger cell. By some unknown reason, the prokaryotic organelle was not consumed. Such lack of consumption would later lead to both cells forming a mutualism, receiving surviving benefits from each other. Over time, the newly combined organelle would develop into the complex eukaryotic cell of today. The endosymbiotic theory has been widely accepted as one of the possibilities of the origins of mitochondria, chloroplasts, and other eukaryotic organelles and cells.

Mitochondria and Chloroplasts

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Mitochondria and chloroplast are two organelles found in eukaryotic cells. Chloroplast is only found in plants while majority of eukaryotic cells have mitochondria. Even though both organelles are found in eukaryotic cells, both mitochondria and chloroplast have characteristics often found in prokaryotic cells.

These observed characteristics of both, mitochondria and chloroplast, are heart of the Endosymbiotic Theory.

A comaparison in chloroplasts and Mitochondria: Both chloroplasts and mitochondria generate ATP by chemiosmosis, but they use different sources of energy. Mitochondria transfer chemical energy from food to ATP. Chloroplasts transform light energy into the chemical energy of ATP. Mitochondria can be found in animal cells and Chloroplasts can be found in plant cells. In mitochondria, protons are pumped to the intermembrane space and drive ATP synthesis. In chloroplasts, protons are pumped into the thylakoid space and drive ATP synthesis as they diffuse back into stroma.

In 1883, French botanist Andreas Franz Schimper (1856–1901) observed that the division of chloroplasts was similar to that of the free-living cyanobacteria. Schimper would later propose in a footnote that symobiotic union of organisms lead to the evolution of green plants. He was the first to study and describe the potential endosymbiotic nature in these cells.

While conducting research on lichen, Russian biologist and botanist Konstantin Mereschcowsky (1855–1921) formulated the symbiogenesis theory. In 1905, he first suggested the idea of plastids originating as endosymbionts, which argued that symbiosis was the main driving force of evolution. Mereshcowsky published his finding of mitochondria in his 1926 work, Symbiogenesis and the Origin of Species in collaboration with Ivan Wallin. Mereschocowky proposed that smaller and less complex cells formed symbiotic relationships with larger complex cells. Mereshcowsky believed that many large complex cells like chloroplasts evolved through this process.

Ivan Wallin was the first to suggest that eukaryotic cells were composed of microorganisms. Alongside Mereschocowsky, he published his works in "Symbiogenesis and the Origin of Species".

American biologist Ivan Emanuel Wallin (1883–1969) proposed, after studying and working with mitochondria, that species derived from bacteria have origins in endosymbiosis. He was the first to suggest the idea that the eukaryotic cell was composed of microorganisms. This lead to the formation of the Endosymbiotic Hypothesis. Wallin published his findings in his 1926 work, Symbiogenesis and the Origins of Species, alongside Mereschocowsky, where they formulated their ideas of symbiogenesis. Their theories were originally rejected due the assumption that mitochondria and chloroplasts did not contain DNA. However, this was proven false during the 1960s, when Hans Ris revived the theory.

In 1981, Dr. Lynn Margulis contributed to the endosymbiosis theory with the publication of her work, Symbiosis in Cell Evolution. Her research claimed that the origin of mitochondria were separate organisms that originally entered into a symbiotic relationship with eukaryotic cells through endosymbiosis. This became the primary support for the endosymbiotic theory, causing her to became the leading figure behind the endosymbiotic hypothesis.

Margulis essentially argued against the idea of random mutation, which was accepted as the main source of genetic variation with species. Instead she thought a symbiotic merger played a much larger role in the creation of new genomes and genetic diversity. She believed that instead of mutations, DNA in the cytoplasm of cells originated from the genes of prokaryotes(bacteria) that had become organelles.

Dr. Lynn Margulis continued to study the origins of mitochondira and chloroplast in eukaryotic cells during her time at University of Massachusetts Amherst. She discovered that these organelles originated as prokaryotic endosymbionts that later started to show in eukaryotic cells. Margulis showed convincing research evidence that mitochondria evolved from aerobic bacteria called Proteobacteria, and chloroplasts evolved from endosymbiotic cyanobacteria. Her research was published in her work “Symbiosis in Cell Evolution” (1981).

Dr. Margulis proposed that eukaryotic flagella and cilia originated from endosymbiotic spirochetes. Due to lack of DNA and the fact that they do not show any ultrastructural similarities to prokaryotes, there is not enough evidence to support this claim. Even though DNA is not present, peroxisomes are considered to be a consequence from the origin of endosymbiotic. As a matter of fact, the original emdosymbionts were projected by Christian de Dave himself.

Considering Darwin’s idea of evolution, Margulis and Sagan believed, ‘Life did not take over the globe by combated, but by networking,’ for example, by cooperation, interaction, and mutual dependence between living organisms.

Dr. Margulis was awarded the National Medal of Science by President Bill Clinton in 2000, for her amazing work on the endosymbiotic hypothesis. She was also a member of the Russian Academy of Natural Science and the National Academy of Sciences. E.O Wilson has titled her ‘one of the most successful synthetic thinkers in modern biology’. Her work has considerably helped promote the study of endosymbiosis from a hypothesis to theory.

While studying the structure of cells in the late 1960s, American biologist Lynn Margulis (1938- ), noticed, like many before her, that mitochondria have similar characteristics to bacteria. She would argue in her 1966 work, The Origin of Mitosing Eukaryotic Cells, that the major force of cell evolution is symbiosis. Her 1966 work would become a landmark for the modern endosymbiotic theory. The idea of endosymbiotic theory was that multiple prokaryotic organism were engulfed by one another. Booth survived and evolved to eukaryotic cells over a period of a millions of years. Margulis would later connect this theory with her previous work in her 1970 book, Origin of Eukaryotic Cells. Margulis’s theory gained support in the 1980s after research showed that the genetic material of mitochondria and chloroplasts differed from that of nuclear DNA. The theory has also been accepted as the theory of evolution for some organelles.

Endosymbiotic Theory proposes that mitochondria developed from proteobacteria, or Rickettsia, (respiring Bacteria) and that chloroplasts were originally from cyanobacteria.
Anaerobic bacteria engulfed the aerobic bacteria, which was not completely digested. They formed a symbiotic relationship, and both cells benefited from each other mutually. The anaerobic bacteria procured food for the aerobic bacteria and provided it with a safe shelter. The engulfed bacteria was able to provide a new aerobic method for converting oxygen that was toxic to the anaerobic bacteria into ATP. Aerobic bacteria eventually became mitochondria, which is supported by the fact that mitochrondria have their own cellular mechanisms and DNA. In addition to these cells, the heterotrophic host cell also engulfed photosynthetic cyanobacteria, which were able to use photosynthesis to convert sunlight into ATP. This additional trait was beneficial for the newly formed eukaryotic cell. This symbiont cell became the chloroplast. Over numerous years, mitochondria and chloroplast became gradually more specialized to such an extent that they now cannot survive outside of the cell. These cells had a greater advantage over other cells, and, through natural selection, became more prevalent than other types of cells. Eukaryotes with only mitochondria became animal and fungi cells, whereas those with both mitochondria and chloroplasts became plants cells.

Modern scientists believe that certain organelles found exclusively in eukaryotic cells may have arisen from various prokaryotic ancestors that initiated endosymbiotic relationships with host cells that consumed them. This belief is supported by evidence such as the discovery of DNA and ribosomes within organelles like mitochondria and chloroplasts. Examining the DNA has led scientists to find similarities in the sequences to those of modern living bacteria. For example, each organelle in eukaryotes has a single, circular DNA that is more similar to prokaryotes than eukaryotes. In addition, transfer RNA, ribosomes, and other molecules involved into transcription and translation processes were examined and compared with prokaryotes; it was discovered that they are similar in term of nucleotide sequence, size and even sensitivity to certain antibiotics. Furthermore, the existence of double membranes over many of these organelles suggests the possibility that the inner membrane may have belonged to the original prokaryote while the outer membrane may have formed from food vacuoles as the host cell devoured the prokaryote. The inner membrane of these organelles contains enzymes and transport systems that are similar to the plasma membrane of prokaryotes. In addition, certain species of modern organisms such as amoebas have been known to live, via endosymbiosis, with aerobic prokaryotes.

Mitochrondria and chloroplasts also replicate by a splitting process similar to binary fission in prokaryotes.

Based on the nucleotide sequences on RNA of organelles such as mitochondria and chloroplasts, systematists found similarities between mitochondria and alpha proteobacteria and chloroplasts and cyanobacteria. They concluded that alpha proteobacteria gave rise to mitochondria and cyanobacteria gave rise to chloroplast in eukaryotes.

It appears that over time, there has been a transfer of genes from the mitochondrial genome to the nuclear genome. The ancestral bacterial genes that were thought to be part of the mitochondria have been found in the nuclear genome. Evidence for this hypothesis can be seen from orthologous genes. In some species, the orthologous genes can be found in the mitochondria, while it is found within the nuclear genome of other species.

This hypothesis of gene transfer from the mitochondria to the nucleus explains why proteins necessary for mitochondrial DNA (mtDNA) replication are not coded in the mitochondria. In fact, proteins for mtDNA replication, including transcription and translation, are coded for in the nucleus of the cell.

Despite the transfer of genes between the mitochondria and nucleus, the mitochondria has still retained much of its own independent genetic material. Speculation has occurred as to why the mitochondria is still necessary for maintaining complex enzymatic processes, resulting in complicated genomes that may only code for a few genes.

One possible explanation is that it may be difficult to transport hydrophobic proteins across the mitochondrial membrane, and then ensure that they are shipped to the correct location. This explanation suggests that these proteins may be necessary to be produced within the mitochondria. Another possible explanation is that there are differences in codon usage between the nucleus and mitochondria, making it difficult to be able to fully transfer the genes. A third possible explanation is that the mitochondria needs to produce its own genetic material so as to ensure metabolic control in eukaryotic cells. This indicates that mitochondrial DNA may directly have an effect on the respiratory chain and the reduction/oxidation (redox) processes of the mitochondria.